Understanding Physics: Solids, liquids and gases: Edexcel IGCSE Science Double Award Explained

Solids, liquids and gases might sound like the simplest topic in physics, but it connects particle behaviour to density, pressure and the gas laws in ways that the exam loves to test. Here is everything the Edexcel IGCSE Science Double Award expects you to know.

The edexcel igcse science double award physics: solids, liquids and gases section of the 4SD0 specification covers three topics: units, density and pressure, and ideal gas molecules. This is a section where the theory is grounded in everyday experience. You already know that ice floats, that tyres inflate and that deep water exerts more pressure than shallow water. What the specification adds is the physics underneath those observations. These edexcel igcse science double award revision notes will walk you through each topic so the exam holds no surprises.

Units

Pay attention to unit conversions. Density is sometimes given in g/cm³ rather than kg/m³. To convert: 1 g/cm³ = 1000 kg/m³. Temperature for gas law calculations must be in kelvin: K = °C + 273. These conversions are straightforward, but forgetting them under exam pressure is a common source of error.

Density and pressure

Density is the mass per unit volume of a substance: ρ = m / V. A dense material packs more mass into a given volume. Steel has a high density (about 7800 kg/m³); air has a very low density (about 1.2 kg/m³ at sea level). An object floats in a fluid if its density is less than the fluid's density. This is why ice floats on water, and why a helium balloon rises through air.

To measure the density of a regular solid, measure its mass on a balance and calculate its volume from its dimensions (length x width x height for a cuboid). For an irregular solid, use a displacement method: lower the object into a measuring cylinder or eureka can of water and record the volume of water displaced. For a liquid, measure a known volume in a measuring cylinder and find its mass on a balance.

Pressure is the force per unit area: P = F / A. Pressure is measured in pascals (Pa), where 1 Pa = 1 N/m². The same force applied over a smaller area produces greater pressure. This is why a drawing pin has a sharp point: the small area at the tip concentrates the force, making it easy to push into a noticeboard. Snowshoes work on the opposite principle, spreading your weight over a larger area to reduce the pressure on the snow so you do not sink in.

Pressure in a liquid: The pressure in a liquid increases with depth. At any given depth, the pressure acts equally in all directions. The equation is: P = ρgh, where ρ is the density of the liquid, g is gravitational field strength (10 N/kg) and h is the depth below the surface.

Hydraulic systems: Pressure in a liquid is transmitted equally in all directions. This principle is the basis of hydraulic systems, used in car brakes, hydraulic jacks and heavy machinery. A small force applied to a small-area piston creates a pressure that is transmitted through the fluid to a large-area piston, producing a larger force. The pressure is the same throughout the fluid, but because F = PA, a larger area means a larger output force. This is how a small foot pressure on a brake pedal can generate enough force to stop a car.

Atmospheric pressure: The atmosphere exerts pressure on everything at the Earth's surface. This pressure is caused by the weight of the air above us. At sea level, atmospheric pressure is approximately 101,325 Pa (about 100 kPa). Atmospheric pressure decreases with altitude because there is less air above you as you go higher. This is why your ears pop on an aeroplane and why mountaineers at high altitude experience lower air pressure.

Ideal gas molecules

The behaviour of gases is explained by the kinetic theory of matter. In a gas, particles are widely spaced, move rapidly in random directions, and collide with each other and with the walls of their container. These collisions with the container walls create pressure. The edexcel igcse science double award specification treats gases as "ideal," meaning the particles themselves have negligible volume and the only interactions between them are elastic collisions (no energy is lost).

Three gas laws describe how pressure, volume and temperature are related:

Boyle's law: At constant temperature, the pressure of a gas is inversely proportional to its volume. P₁V₁ = P₂V₂. If you compress a gas (decrease its volume), the particles have less space to move in, so they hit the walls more often, and the pressure increases. If you expand the gas, the opposite happens.

Charles's law: At constant pressure, the volume of a gas is directly proportional to its absolute temperature (in kelvin). V₁/T₁ = V₂/T₂. Heating a gas gives the particles more kinetic energy, so they move faster and push the walls outward, increasing the volume (if the pressure is free to stay constant).

Pressure law: At constant volume, the pressure of a gas is directly proportional to its absolute temperature. P₁/T₁ = P₂/T₂. Heating a gas in a sealed container makes the particles move faster and hit the walls harder and more frequently, increasing the pressure.

All three laws can be combined into the combined gas equation: P₁V₁/T₁ = P₂V₂/T₂. This is useful when pressure, volume and temperature all change simultaneously.

Absolute zero: The kelvin scale starts at absolute zero (0 K, which is -273 degrees Celsius). At absolute zero, particles have the minimum possible kinetic energy and (in an ideal gas) would stop moving entirely. No temperature lower than absolute zero can exist. The kelvin scale is essential for gas law calculations because the gas laws require proportional relationships, and the Celsius scale does not start at zero energy.

The igcse 4sd0 physics: solids, liquids and gases specification also asks you to explain changes in gas behaviour using kinetic theory. When the exam asks "explain why..." for a gas law question, describe what happens to the particles: how fast they move, how often they collide with the walls, and how hard those collisions are. Simply stating the law itself does not answer an "explain" question.

Exam preparation

The physics: solids, liquids and gases edexcel igcse practice questions in the edexcel igcse science double award come in two flavours: calculations and explanations. Calculation questions test whether you can apply the density, pressure and gas law equations correctly. Explanation questions test whether you understand the kinetic theory well enough to describe particle behaviour in your own words.

Self-check questions

1. State two differences between the structure of a plant cell and an animal cell.
2. Describe the process of osmosis and explain why it is important in living organisms.
3. Explain the difference between an exothermic and an endothermic reaction, giving one example of each.
4. A car accelerates from rest to 20 m/s in 5 seconds. Calculate its acceleration and the resultant force if the car has a mass of 1200 kg.
5. Describe the structure of an atom, naming each subatomic particle and stating its relative charge and mass.

Study these edexcel igcse science double award notes with both types in mind. Practice the calculations until the equations feel automatic, and practice the explanations until you can describe particle behaviour without hesitating. Use the Green Bridge CBT platform for edexcel igcse science double award practice questions and edexcel igcse science double award revision notes and past questions on this topic. The combination of formula fluency and particle-level understanding is what turns a good answer into a full-marks answer.